Author: Xiaolei Zhang
Lead Author’s Institution: Department of Physics and Astronomy, George Mason University
Status: Submitted to Earth and Planetary Science Letters [open access]
Author’s note: The discussion in this astrobite was written from the point of view that the hypothesis in the paper is correct – which of course remains to be seen by future studies.
Science requires collaboration, and often that collaboration leads to projects that span across different fields of science. This paper caught my eye mainly because the Colorado Plateau is one of my favorite places to travel, and though I’m not a geologist, its unique and beautiful rock formations have always piqued my interest. As I read this paper, I was surprised to find that this author also was not a geologist by trade. Xiaolei Zhang is an astrophysicist, but decided to carry on her late father’s unfinished project to uncover the formation of the Colorado Plateau and the possibility of giant-impact-plate tectonic motion.
By looking at planetary geology through an astronomer’s lens, it is clear that though discoveries of exo-solar planets are exciting, sometimes discoveries about humanity’s first known planet can be just as intriguing. There are still many things to learn about our home planet Earth. For example, geographical features on Earth can tell us about its astronomical history— its track record with meteors and other space debris.
Today’s paper discusses evidence Earth collided with a Mars-sized object roughly 750 Million years ago to form today’s Colorado Plateau. To put this in perspective, the impact that contributed to the mass extinction of dinosaurs was likely caused by an asteroid only about 10 km across.
The Colorado Plateau is the region around the Four Corners region of the southwestern United States. It is about 337,000 square kilometers and consists mostly of a high-desert plateau of a shallow bowl shape. The Plateau is a surprisingly stable region; the entire area does not have fault lines or features that indicate strong plate movement, which is a bit unusual given that its surroundings have these features. Sedimentary record shows that the entire North America continent, including the Colorado Plateau region, was very flat in early times, but now it has many mountains and sharp geographic features surrounding the area. These geographical peculiarities point towards an impact with an astronomical object significantly different from Earth forming this plateau.
The features discussed above indicate that this region was very likely formed by an impact, and since we are able to know the age of some of the features originating from the impact, we know at what time this collision happened. Zhang discusses that this impact likely happened 750 million years ago.
At this point in time, out solar system would have been very stable in its structure, so if an impact occurred, where could this object have come from? Zhang suggests the impacting object came from a spiral density wave as the solar system orbited the Milky Way; as the solar system orbited the Milky Way, it could have passed through a spiral arm of the galaxy. If this happened, a free-floating Mars exoplanet could have been drawn into the solar system and then crashed into Earth.
Zhang discusses multiple geographical features that provide evidence for the giant-impact theory about the Colorado Plateau. She also provides quantitative arguments to support the claim of the Mars impact. To review, these arguments include
- Despite much igneous and orogenic activity at its boundaries, the plateau has remained structurally stable.
- The type of rock throughout this area indicate evidence of shock metamorphism.
- The basalt throughout the lowest level of rock also shows shock geographic formation. These features can also be dated around the time of the theorized impact.
This rare paper combines both geology and astrophysics spectacularly. By using the geological evidence and providing an astronomical theory, Zhang shows in a unique way how our planet got some of its features. Using interdisciplinary studies like this, we could learn much more about other planets within our solar system and newly-found exoplanets.
Zhang presents a quantifiable theory of a giant impact formation of the Colorado Plateau; though others have qualitatively mentioned this theory, this paper is the first to give a quantifiable estimate of size based on collisional dynamics. Zhang successfully completed this geological project by approaching it from her astrophysicist point of view, which is an incredibly important part of interdisciplinary science. To me, this project exemplifies the collaborative nature of scientific work; not only does it span across different studies, it spans beyond the lifetime of just one person.